This invention relates to impactless electronic printing method and apparatus. Specifically, the invention relates to an electronic xerographic printer having a non-electronic overprinting capability known as a "form overlay" feature.
Form overlays are particularly suited for printing tasks where variable data or images are to be added to fixed data or images. For example, the thousands of bank statements provided to depositors are fixed images to which the variable debit and credit data of the many individual depositors is added or integrated. However, the bank statement printing chore is comparatively straight forward because the form contains blank or white spacers for receiving the variable debit and credit data. That is, the variable data doesn't overlie the fixed data. A more complex task is overprinting a travelers route onto a complex map where two images or sets of data are printed in the same area on the paper or other support member.
The forms overlay technique is advantageous to electronic printing and computer printing in particular because its use saves the electronic memory storage space required to electronically print the form for other electronic control functions. Often this saving in memory space yields a meaningful cost improvement to the entire printing system.
Prior art forms overlay electronic impactless printers are negative on negative printers. The Xerox Corporation Xerox 1200 Computer Printer, the Siemens Corportion Siemens 3352 Laser Printer and the International Business Machine Corporation IBM 3800 Printing Subsystem are examples of negative on negative or negative overlay electronic printers. Briefly, in each, a latent electrostatic image is generated in response to electronic signals on a charged photoconductive surface by exposing the charged surface to electromagnetic radiation (hereinafter "light") thereby discharging the surface in areas struck by the light. The discharged areas against the undischarged background define the negative latent electrostatic image. (The inverse defines a positive latent electrostatic image.) The discharged areas are analogous to white markings on a black background for a reflection image or transparent markings on an opaque background for a transparency image. The negative latent image is made visible or developed by depositing black toner onto the discharged areas and transferring the black toner to white paper. The development and transfer steps convert the negative latent electrostatic image to a positive reflection image. The toner image printed from the electronic signals is herein referred to as an electronic image or print.
The "form overlay" print or image is created by the above commercial computer printers by exposing the charged photoconductive surface to a light image of a negative reflection print or negative transparency print or overlay. When the negative overlay is flooded with light and the reflected or transmitted light strikes the photoconductor a negative latent electrostatic image is created. The overlay latent image is composed of discharged areas representing markings on a charged background. The development and transfer of the overlay latent image inverts or converts the negative sense of the image to the positive sense as in the case of the above described electronic image.
The overlay and electronic images are generated during a single cycle of the above xerographic computer printers. Each of these printers employ a rotating photoconductive drum and the electronic and overlay images are generated during a single cycle or rotation of the drum prior to the development step. This means that one image is superimposed over the other on the same area of the photoconductive drum. This superimposition is possible because of the negative mode of operation. The first image, e.g. the electronic image, discharges the drum only in information areas and not background areas. Statistically, the information areas occupy significantly less space than the background areas. Consequently, the second image, in this example the overlay image, is recorded in the charged background regions of the electronic image. In other words, the second image can be produced only if the first left significant areas of charge on the drum surface on which the second image can be recorded.
The foregoing explains one reason why a positive form overlay capability analogous to the negative form overlay is not used in a positive electronic image recorder such as that of the Xerox Corporation 9700 Computer Printer. In the Xerox 9700 machine, the discharged regions of a photoconductive member (a belt in this machine) are the background areas and the charged regions are the information areas. Here, once the electronic image is created, there is effectively no charged region on the drum in which to record the subsequent overlay information except that information areas of the electronic image which is normally inadequate. Also, writing the overlay image in the information region of the electronic image is likely to destroy its content.
A significant disadvantage of prior art negative on negative overlay computer printers is the fundamental problem that negative prints are expensive if for no other reason than their unavailability. The majority of the world's images are black marking on white backgrounds, i.e., positive images. This disadvantage is amplified when an electronic printer is employed in an application where many different forms are to be printed on by variable information. Also, when the original document is a color image the negative image sense of it may not be appropriate for an overlay.